An optical film is a film capable of transmitting, reflecting, or absorbing light, and can exhibit optical functions such as refraction, birefringence, antireflection, viewing angle expansion, light diffusion, and luminance enhancement.
Optical films are used in flat panel displays (FPD) such as liquid crystal displays (LCD) and plasma displays (PDP), and in windowpanes for buildings and vehicles, as an infrared shielding film, an ultraviolet shielding film, an antireflective film, an oriented film, a polarizing film, a polarizing plate protective film, a retardation film, a viewing angle expansion film, a luminance improving film, an electromagnetic wave shielding film, and the like.
An example of the optical film may be a film in which at least two or more optical functional layers are formed and used. Regarding the optical film, for example, a film having a multilayer laminated film structure in which high refractive index layers and low refractive index layers are laminated on the surface of a base material, with the optical film thicknesses of the layers being respectively adjusted, is known. Such a film is known to selectively reflect light having a particular wavelength, and an optical film having a multilayer laminated film structure is used as an optical reflective film.
In an optical reflective film, the reflection wavelength can be controlled only by adjusting the film thicknesses or the refractive indices of respective layers. Thereby, the optical reflective film can selectively reflect infrared radiation, ultraviolet radiation, or visible light, as a result of the design according to the purpose of use of the film.
Above all, particularly, as the public interest on energy saving measures of recent years is ever increasing, from the viewpoint of reducing the load imposed on air-conditioning facilities, there is an increasing demand for an infrared shielding film that is attached on a windowpane of a building or a vehicle and blocks transmission of heat rays of sunlight.
Regarding a method for forming an infrared shielding film, methods of forming a laminated film having a configuration of alternately laminating high refractive index layers and low refractive index layers, using a dry film-forming method such as a vapor deposition method or a sputtering method, have been mainly proposed. However, in dry film-forming methods, the vacuum apparatuses and the like that are used for film formation need to become large-sized, the method requires high production cost, it is difficult to obtain a film having a large area, and the base material is limited to heat-resistant materials only.
In place of the dry film-forming methods having such problems as described above, a method of forming an infrared shielding film using a wet coating method is known (see, for example, JP 2009-86659 A).
Generally, methods for producing a laminated film having two or more layers on a base material by wet coating include sequential coating application of applying, drying, and laminating layer by layer; and simultaneous multilayer coating application of simultaneously applying a plurality of layers. Examples of sequential coating application include a spin coating method, a bar coating method, blade coating application, and gravure coating application; however, particularly in a case in which a multilayer film such as an optical reflective film is produced, since the number of times of coating application and drying processes becomes large, productivity is low. On the other hand, examples of simultaneous multilayer coating application include methods using curtain coating application and slide bead coating application, and since a plurality of layers can be formed, productivity is high. Therefore, simultaneous multilayer coating application is preferably employed.